The top U.S. commander in the Middle East estimates that additional U.S. troops could arrive in Afghanistan within days or weeks, but, according to several U.S. officials, Defense Secretary James Mattis has yet to sign any orders to deploy more forces and is reviewing the Pentagon's earlier force recommendation.

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General Joseph Votel’s comments -- first reported by the Associated Press as the head of U.S. Central Command traveled in Saudi Arabia -- were confirmed by a U.S. official.

Votel, who was in Afghanistan over the weekend, said what's most important for the U.S. military "is to get some capabilities in to have an impact on the current fighting season," referring to the Taliban's spring fighting season, which began in April.

But several U.S. officials tell ABC News that those forces will take time to deploy, and that Mattis is still reviewing the exact number of troops that will go to Afghanistan.

In a speech on Monday, President Trump suggested additional troops would be sent to the region, but did not reveal specific numbers. Instead, Trump said, "Conditions on the ground, not arbitrary timetables, will guide our strategy from now on."

Mattis has favored the Pentagon's recommendation of 3,900 additional forces, but while traveling in Iraq on Tuesday he told reporters that the troops deployed "may or may not be the number that's bandied about."

Mattis said once he has the Chairman of the Joint Chiefs of Staff's plan for Afghanistan, "I'll look at the number we have on the ground, reorganize those on the ground to align with the new strategy, and bring whatever gap fillers I need."

Additional troops will not go to Afghanistan until Mattis signs formal deployment orders, which has not happened yet, officials said.

They added that while the Joint Staff has identified certain units that might be deployed to Afghanistan, those units have not received formal preparation orders.

An estimated 8,400 U.S. troops are serving in Afghanistan in an advisory capacity or in counter-terrorism operations against the Taliban and ISIS-Khorasan, the ISIS affiliate in the country.